NCERT Class 12 Biology Ch 9 Biotechnology Solutions PDF

Concept used. A recombinant protein is any protein produced by expressing a cloned gene in a heterologous host (typically E. coli, yeast, or mammalian cell lines). Because the gene is inserted into a host through recombinant DNA technology, the protein it codes for is identical to (or a deliberately modified version of) the human/animal protein, but can be manufactured in industrial quantities at controlled purity. Medical recombinant proteins replace a missing endogenous protein, neutralise a pathogen, or act as a biological signal.

1. Ten recombinant therapeutic proteins commonly named in NCERT-aligned references:
   2. Human Insulin (Humulin) — treats type-1 and advanced type-2 diabetes mellitus.
   3. Human Growth Hormone (Somatotropin / hGH) — treats pituitary dwarfism and Turner syndrome.
   4. Erythropoietin (EPO) — treats anaemia in chronic kidney disease and chemotherapy patients.
   5. Interferon α, β, γ — antiviral therapy in chronic hepatitis B/C, and disease-modifying therapy in multiple sclerosis.
   6. Tissue Plasminogen Activator (tPA / Alteplase) — clot-buster used within hours of an acute ischaemic stroke or myocardial infarction.
   7. Factor VIII — corrects clotting deficiency in haemophilia A.
   8. Factor IX — corrects clotting deficiency in haemophilia B (Christmas disease).
   9. Hepatitis B surface antigen (HBsAg) vaccine — prevents hepatitis B infection; the first widely used recombinant vaccine.
   10. Granulocyte Colony Stimulating Factor (G-CSF / Filgrastim) — boosts neutrophil counts after chemotherapy.
   11. Monoclonal antibodies such as Trastuzumab (HER2-positive breast cancer) and Rituximab (non-Hodgkin lymphoma, rheumatoid arthritis).
2. Why this list matters. Each protein here was first purified in milligram amounts from human/animal sources at prohibitive cost. Recombinant production in microbial or mammalian bioreactors dropped the cost per dose by 10–100× and removed the risk of carrying through donor-derived viruses (HIV, hepatitis) that haunted plasma-derived clotting factors in the 1980s.

Ten recombinant therapeutic proteins: insulin, hGH, EPO, interferons, tPA, Factor VIII, Factor IX, HBsAg vaccine, G-CSF, monoclonal antibodies (e.g. Trastuzumab, Rituximab).

Concept used. A restriction endonuclease is a bacterial enzyme that scans double-stranded DNA, recognises a specific short palindromic sequence (usually 4–8 base pairs), and cleaves the sugar–phosphate backbone of both strands inside or adjacent to that recognition site. The most studied member is EcoRI from Escherichia coli, which recognises the hexamer 5'-GAATTC-3' and cuts between G and A on each strand, leaving 4-nucleotide single-stranded overhangs called sticky ends. Two DNA fragments cut by the same enzyme have complementary sticky ends and can therefore be joined by DNA ligase.

1. Substrate. Any double-stranded DNA molecule containing the hexanucleotide recognition site. Example below: a plasmid (vector) and a foreign DNA fragment, both containing one 5'-GAATTC-3' site.
2. Recognition site (palindrome). Reading 5'→ 3' on either strand gives the same sequence: arrayc 5' G A A T T C 3'
   3' C T T A A G 5' array
3. Site of cleavage. EcoRI cuts the phosphodiester bond between G and A on each strand, indicated by the arrows below: arrayc 5' GA T T C 3'
   3' C T T A A5' array Because the cuts are staggered (not directly opposite each other), each fragment carries a four-base single-stranded 5'-overhang.
4. Products. Two DNA fragments with complementary 5'-AATT-3' sticky ends: 5' G AATT C 3' 3' CTTAA G 5' Mixing vector-cut DNA with foreign-DNA-cut fragments allows their AATT overhangs to base-pair; DNA ligase then seals the nicks to form a single recombinant DNA molecule.

producing sticky ends and the resulting recombinant DNA.

EcoRI recognises 5'-GAATTC-3', cuts between G and A on both strands, and produces DNA fragments with 4-base 5'-AATT sticky ends that ligate to give recombinant DNA.

Concept used. Molecular size can be compared two ways: by molecular weight (number of atoms / mass in daltons) or by physical length of the molecule. For biological macromolecules a genome's DNA is typically thousands to billions of base-pairs long, while a single enzyme is a folded polypeptide of a few hundred amino acids. Gel-electrophoresis behaviour (Section 9.3.1 of the chapter) is the direct evidence: in an agarose gel the DNA migrates more slowly and gets caught higher up, while a protein of comparable nominal mass slips through.

1. Compare on the same gel. In agarose-gel electrophoresis, DNA fragments and proteins behave very differently. DNA, being uniformly negatively charged and much larger, migrates only short distances and is held back in the gel matrix; proteins of a few tens of kDa run far toward the electrode. The direct visual evidence: DNA stays near the well, the protein runs away.
2. Count the monomers. A bacterial chromosome is ∼ 10⁶–10⁷ nucleotide pairs long. Even the largest enzymes are ≤ 10³ amino acids long. By monomer count alone, DNA wins by 3–4 orders of magnitude.
3. Translate to mass. Average nucleotide pair ≈ 650 Da; average amino acid ≈ 110 Da. So 1 million bp of DNA ≈ 6.5 × 10⁸ Da, whereas a 300-aa enzyme ≈ 3.3 × 10⁴ Da. The DNA is ∼ 2 × 10⁴ times heavier than the enzyme.
4. Visual evidence in the chapter. Section 9.3 figures show one tiny restriction enzyme nibbling at a much longer DNA strand. The enzyme docks onto a 6-bp recognition site that occupies a vanishing fraction of the substrate molecule.

DNA is far bigger than enzymes — by mass (10⁴–10⁵ times) and by physical length (kilobases vs nanometre-scale folded proteins). The proof: on agarose-gel electrophoresis the DNA stays near the well while a ∼ 30 kDa enzyme migrates far down.

Concept used. Molar concentration (molarity, M) is defined as the number of moles of a substance per litre of solution: M = nV = NN_A · V, where N is the number of molecules of the substance, N_A ≈ 6.022 × 10²³ mol^-1 is Avogadro's number, and V is the volume of solution in litres. A diploid human somatic cell contains two copies of the haploid genome (one set of 23 chromosomes from each parent), so N = 2 molecules of nuclear DNA per cell. The remaining unknown is the cell volume, which for a typical mammalian cell is ∼ 1000 ³ = 1000 × 10^-15 L = 10^-12 L.

1. Count the DNA molecules per cell. A diploid human cell contains 46 chromosomes = 2 complete genomes (one maternal, one paternal). Treating each genome as one DNA molecule (it is actually 23 linear molecules, but the question asks for ``human DNA'' as a whole): N = 2 copies.
2. Convert to moles. Number of moles of DNA per cell: n = NN_A = 26.022 × 10²³ = 3.32 × 10^-24 mol.
3. Estimate cell volume. A typical mammalian cell has a diameter ∼ 10–20 . Take V ≈ 1000 ³: V = 1000 × (10^-6 m)³ = 10^-15 m³ = 10^-12 L. (Recall 1 m³ = 1000 L.)
4. Apply the molarity definition. M = nV = 3.32 × 10^-24 mol 10^-12 L = 3.32 × 10^-12 mol/L. In SI prefixes, M ≈ 3.3 pmol/L, i.e. ∼ 3 picomolar.

Molar concentration of human DNA in a single diploid cell ≈ 3.32 × 10^-12 M (∼ 3 picomolar).

Concept used. A restriction endonuclease is a sequence-specific enzyme that cleaves foreign double-stranded DNA at a palindromic recognition site, while the host's own DNA is protected by methylation at the same sites. This pair of activities (restriction + modification) constitutes a prokaryotic defence system against invading viruses (bacteriophages): phage DNA is unmethylated, the host enzymes shred it; the host's own chromosomal DNA carries methyl groups on key bases (typically the adenine of GATC or the cytosine of CCGG) and therefore escapes cleavage. Eukaryotic cells use a fundamentally different anti-viral defence (RNA interference, type-I interferon signalling, restriction factors of the APOBEC/TRIM family) and do not encode the classical sequence-specific restriction enzymes used in genetic engineering.

1. Direct answer. No. Eukaryotic cells do not possess restriction endonucleases of the type used in recombinant DNA technology (Types I, II, III).
2. Justification — why prokaryotes have them. Restriction-modification systems evolved as a microbial immune system against bacteriophages. Bacteria carry a methylase and a restriction enzyme that share the same recognition site: the methylase tags host DNA so it is ignored; the restriction enzyme cleaves any unmethylated invading phage DNA into fragments.
3. Justification — why eukaryotes do not need them. Eukaryotic cells face viruses but defend through different mechanisms: RNA interference (siRNA / miRNA pathway targeting viral mRNA), interferon-induced antiviral state, cell-autonomous restriction factors (APOBEC3, SAMHD1, TRIM5), and the adaptive immune system in metazoans. Carrying promiscuous DNA-cutting enzymes in the cytoplasm would risk fragmenting their own enormous nuclear genome.
4. Caveat — eukaryotic nucleases that cut DNA. Eukaryotes do encode other nucleases: endonuclease G (apoptotic DNA fragmentation), DNase I/II (extracellular digestion), Cas9 (in archaea; introduced into eukaryotes for genome editing). These are not sequence-specific restriction enzymes — they cut after a defined biological trigger (apoptosis, RNA guidance), not at a palindrome.

No. Eukaryotic cells lack the bacterial restriction-modification systems used in cloning. The enzyme evolved as a phage-defence tool in prokaryotes; eukaryotes use RNA interference and the immune system instead.

Concept used. A shake flask is a small (50 mL to 2 L) Erlenmeyer-flask culture agitated on an orbital shaker; it supports research-scale growth but offers no real-time monitoring or control. A stirred-tank bioreactor is an industrial-grade vessel (100–1000 L, sometimes much larger) equipped with an internal stirrer, an aeration sparger, and an integrated control system that monitors and adjusts pH, temperature, dissolved oxygen, foam, and substrate feed in real time. Beyond the obvious aeration/mixing advantage stated in the question, the control architecture is what makes bioreactors the only viable platform for industrial-scale recombinant-protein production.

1. Online process monitoring and control. The bioreactor carries probes for temperature, pH, dissolved oxygen, and (often) glucose and cell-density. Each probe feeds a controller that adjusts heater, base/acid addition, sparger flow rate, and feed pumps in real time. A shake flask offers none of this — you sample manually and adjust offline.
2. Foam control. Vigorous aeration in protein-rich media creates foam that traps cells and clogs sparger filters. A bioreactor has a mechanical foam breaker and/or an antifoam-addition system; a shake flask just overflows.
3. Sterile environment maintained at scale. The bioreactor body is built for in-situ steam sterilisation (the ``Steam for sterilisation'' inlet in Fig. 9.7 a) and carries a sterile-air filter on the sparger. Shake flasks rely on a cotton plug and quickly contaminate above 5 L.
4. Sampling ports for at-line analytics. Aseptic sampling ports let the operator pull aliquots without opening the vessel. This is essential for tracking the log/exponential phase so that downstream processing is triggered at peak yield.
5. Scalable working volume (100–1000 L+). Shake flasks plateau at ∼ 5 L; a single bioreactor processes 100–10 000 L per run, giving the biomass needed for commercial recombinant-protein quantities. Production bioreactors for monoclonal antibodies routinely reach 20 000 L.
6. Controlled substrate feeding (fed-batch / continuous). Bioreactors support fed-batch mode (slow addition of glucose to prevent overflow metabolism) and continuous mode (medium added and harvested simultaneously to keep cells in exponential phase indefinitely). Shake flasks are strictly batch — you set up once and harvest at the end.
7. Reproducibility for GMP compliance. The digital-control loop produces a documented batch record (cell density vs time, pH vs time, etc.) that meets Good Manufacturing Practice requirements for therapeutic products. Regulators will not accept shake-flask product for clinical use.

Stirred-tank bioreactors offer online pH/temperature/DO monitoring and control, foam control, sterile in-situ steam sterilisation, aseptic sampling ports, scalability to thousands of litres, fed-batch and continuous feeding, and GMP-grade reproducibility.

Concept used. A palindromic DNA sequence is a double-stranded sequence that reads the same 5'→ 3' on both strands. Because of Watson–Crick base-pairing rules (A pairs with T, G pairs with C), this requires the sequence on one strand to be the reverse complement of itself. Palindromic sites are the recognition sites for nearly all Type-II restriction enzymes used in cloning.

1. Five classical palindromic restriction sites.
   1.25 tabularl l l

   Enzyme & Source organism & Recognition site (5'→ 3')
   

   EcoRI & Escherichia coli RY13 & G A A T T C
   BamHI & Bacillus amyloliquefaciens H & G G A T C C
   HindIII & Haemophilus influenzae Rd & A A G C T T
   PstI & Providencia stuartii & C T G C A G
   SalI & Streptomyces albus G & G T C G A C
   

   tabular

   Each site is six base pairs long and reads identically in the 5'→ 3' direction on both strands.
2. Demonstrate the palindrome property for EcoRI. arrayl l 5' G A A T T C 3' & (forward strand)
   3' C T T A A G 5' & (complement, written 3'→ 5') array Now read the lower strand in the 5'→ 3' direction (i.e. right to left): G-A-A-T-T-C. Identical to the top strand. Therefore 5'-GAATTC-3' is a true palindrome.
3. Construct a new palindrome. Pick the first half freely, then write its reverse complement as the second half. Example construction:
   • Choose first three bases on the top strand: 5'-ATC-3'.
   • Reverse complement of ATC is GAT (complement of A,T,C is T,A,G; reverse it to get G,A,T).
   • Append GAT to the top strand: 5'-ATCGAT-3'.
   Verify: complement is 3'-TAGCTA-5'. Read in 5'→ 3': ATCGAT. Match. So 5'-ATCGAT-3' is a 6-bp palindromic sequence (this is, in fact, the recognition site of ClaI).

Five palindromic recognition sites: EcoRI (GAATTC), BamHI (GGATCC), HindIII (AAGCTT), PstI (CTGCAG), SalI (GTCGAC). A custom palindrome built by base-pair rules: ATCGAT (= ClaI site).

Concept used. Meiosis is the reductional cell division that produces gametes; it consists of two successive nuclear divisions, Meiosis I and Meiosis II, with a single round of DNA replication beforehand. The defining feature of Meiosis I is the formation of bivalents (tetrads), the pairing of homologous chromosomes in Prophase I, and the physical exchange of chromosome segments between non-sister chromatids through crossing over. Crossing over is the natural, intracellular generation of recombinant DNA — and it occurs at the pachytene sub-stage of Prophase I.

1. Set-up — pre-meiotic S phase. Before meiosis begins, each chromosome is duplicated so that every chromosome consists of two identical sister chromatids joined at the centromere.
2. Prophase I — Leptotene. Chromosomes condense and become visible. No recombination yet.
3. Prophase I — Zygotene. Homologous chromosomes pair side-by-side along their length, a process called synapsis. The pair is held together by a protein scaffold (the synaptonemal complex), forming a bivalent of four chromatids (a tetrad).
4. Prophase I — Pachytene (the key stage). Recombination nodules — large protein complexes — assemble on the synaptonemal complex. They catalyse a programmed double-strand break on one chromatid, invasion of the homologous chromatid, and reciprocal exchange of segments between non-sister chromatids of homologous chromosomes. This physical exchange of DNA is crossing over, and the chromatids that result carry recombinant DNA — DNA molecules with sequences from both maternal and paternal origin joined by covalent bonds.
5. Prophase I — Diplotene. The synaptonemal complex dissolves; the homologues partially separate but remain joined at the sites of crossover, now visible as chiasmata. These X-shaped points are the cytological evidence of the recombinant DNA molecules made at pachytene.
6. Independent assortment in Metaphase I + Anaphase I. Bivalents line up at the equator with random maternal/paternal orientation; segregation in Anaphase I separates the recombinant chromosomes into daughter cells, propagating the new combinations into the gametes.

Recombinant DNA is generated naturally during Pachytene of Prophase I of Meiosis I, when non-sister chromatids of homologous chromosomes undergo crossing over, exchanging equivalent segments.

Concept used. Transformation is the uptake of foreign DNA by a host cell. To prove that a host cell has indeed taken up the recombinant plasmid (and not merely survived the treatment), molecular biologists use two markers carried on the vector:

• Selectable marker — typically an antibiotic-resistance gene (e.g. amp^R). Cells lacking the plasmid die on the antibiotic plate; cells carrying the plasmid live. This tells you the cell took up some plasmid.
• Reporter enzyme / gene — a gene whose product is easily and visibly assayed (colour, fluorescence, light, enzyme activity). The reporter is engineered so that the cloning site sits inside the reporter gene; a successful insert disrupts the reporter, producing a detectable colour change.

The classical example is insertional inactivation of β-galactosidase (blue-white screening) using the lacZα gene.

1. The set-up — blue/white screening with lacZ. The cloning vector (e.g. pUC18) carries:
   • An antibiotic-resistance gene (amp^R) as the selectable marker.
   • The lacZα fragment as the reporter, with a multiple-cloning site (MCS) embedded inside it.
   The host E. coli carries the complementary lacZω fragment; together with α they produce active β-galactosidase.
2. Plate on ampicillin + X-gal + IPTG.
   • Untransformed cells ⇒ no amp^R ⇒ die.
   • Transformed cells carrying empty vector ⇒ intact lacZα ⇒ β-galactosidase active ⇒ cleaves X-gal to a blue dye ⇒ blue colony.
   • Transformed cells carrying recombinant vector (insert disrupts lacZα) ⇒ no active β-galactosidase ⇒ X-gal stays colourless ⇒ white colony.
3. Why both markers are needed. The selectable marker (antibiotic resistance) alone tells you which cells took up plasmid, but not whether the plasmid carries your insert. The reporter (lacZ) alone cannot pre-enrich for plasmid-carrying cells — you would be screening millions of background colonies. Combining the two: select on antibiotic, then screen the survivors for the recombinant (white) colonies.
4. Modern reporter enzymes. Beyond lacZ, the same logic uses GFP (green fluorescent protein, visible under UV), luciferase (emits visible light in the presence of luciferin), chloramphenicol acetyltransferase, and β-glucuronidase (GUS) in plants.

A reporter gene (e.g. lacZα) is engineered so that successful insertion of foreign DNA disrupts it; the host loses the reporter activity (no blue colour with X-gal) while the selectable antibiotic-resistance gene still works, giving white colonies on a blue background that are the true recombinants.

Concept used. Each sub-part names a single component of the recombinant-DNA pipeline: the molecular feature that lets the plasmid copy itself (origin), the engineering vessel that lets us grow host cells at scale (bioreactor), and the post-fermentation purification steps that convert culture broth into a marketable product (downstream processing).

(a) Origin of replication (ori). A specific sequence of nucleotides in a vector at which DNA replication initiates. When the host's DNA-replication machinery (DNA polymerase III holoenzyme in bacteria) recognises the ori, it loads on, unwinds the double helix and begins synthesising a new strand bidirectionally. Any piece of foreign DNA ligated downstream of the ori rides along and is amplified to the same copy number as the vector. The ori also determines that copy number: high-copy plasmids (pUC series, ori derived from ColE1) maintain 500–700 copies per cell, while low-copy plasmids (pBR322 derivatives, 15–20 copies) trade yield for stability of large inserts.

(b) Bioreactors. Vessels typically of 100–10 000 L working volume, in which raw materials (medium, substrates, inoculum) are biologically converted into specific products (recombinant proteins, enzymes, secondary metabolites, biomass) by microbial, plant, animal or human cells. A bioreactor provides optimal conditions: controlled temperature, pH, dissolved oxygen, substrate concentration, agitation. The most common design is the stirred-tank bioreactor (Fig. 9.7 a): cylindrical or curved-base vessel with a vertical-shaft impeller, sparger for sterile-air supply, jacketed walls for temperature control, and probes for pH/DO. A sparged stirred-tank bioreactor (Fig. 9.7 b) bubbles sterile air through a perforated ring to multiply oxygen-transfer area.

(c) Downstream processing. After fermentation is complete, the product must be separated from cells/medium and purified before sale. Downstream processing is the collective name for this sequence:

1. Cell separation — centrifugation, microfiltration or settling to recover cells (if intracellular product) or supernatant (if secreted).
2. Cell disruption — sonication, French press, bead-milling or lysozyme treatment if the product is inside the cell.
3. Purification — successive chromatography steps (ion-exchange, affinity, gel-filtration), precipitation, ultrafiltration. Affinity chromatography (His-tag on Ni-NTA, for example) can give ≥ 90% purity in a single step.
4. Formulation — adding stabilisers, preservatives, excipients to the active ingredient and adjusting pH/tonicity.
5. Quality control + clinical trials — strict QC testing for purity, potency, sterility, endotoxin and residual host DNA. For drugs, formal Phase I–III trials.

Downstream processing typically accounts for ≥ 60% of the total manufacturing cost of a recombinant therapeutic.

(a) Origin of replication = vector sequence where DNA replication starts; controls copy number. (b) Bioreactors = engineered vessels (100–10 000 L) for controlled large-scale cell culture. (c) Downstream processing = separation, purification, formulation and QC of the product after fermentation.

Concept used. Each part names a tool of recombinant DNA technology: PCR amplifies a chosen DNA region in vitro; restriction enzymes are sequence-specific molecular scissors for cutting DNA at defined sites; chitinase is the enzyme used to break the chitin-rich cell wall of fungi during DNA extraction.

(a) Polymerase Chain Reaction (PCR). PCR is an in-vitro method to make millions to billions of copies of a chosen DNA region using two short primer DNAs and a thermostable DNA polymerase. A single PCR cycle has three temperature steps:

• Denaturation (∼ 94 ^∘C, 30 s): the double-stranded DNA template melts into two single strands.
• Annealing (∼ 50–65 ^∘C, 30 s): two oligonucleotide primers (typically 18–25 nt long, chemically synthesised, complementary to the flanks of the target region) base-pair with their target sites on each single strand.
• Extension (∼ 72 ^∘C, 30–60 s): thermostable Taq DNA polymerase (from Thermus aquaticus) extends each primer in the 5'→ 3' direction using the provided deoxynucleotides (dNTPs).

Because each new product itself becomes a template in the next cycle, copy number doubles per cycle. After n cycles the amplification factor is 2ⁿ; 30 cycles give 2³⁰ ≈ 10⁹ copies, matching the figure shown in Fig. 9.6.

DNA polymerase. Thirty cycles amplify the chosen DNA region ∼ 10⁹-fold.

(b) Restriction enzymes and DNA. Restriction endonucleases are bacterial enzymes that recognise specific short palindromic sequences (4–8 bp) in double-stranded DNA and cleave both strands. The first one was isolated by Smith & Wilcox (1970) from Haemophilus influenzae and named HindII. By 2024 over 4000 restriction enzymes from > 230 strains of bacteria are commercially available, each with a unique recognition site. They fall into three categories — Type I, II (used in cloning) and III — based on how cleavage and methylation activities are organised. The Type-II workhorses (EcoRI, BamHI, HindIII, PstI, NotI) leave defined sticky ends that ligase can rejoin, making them the cutting tools at the heart of recombinant DNA technology.

(c) Chitinase. An enzyme that hydrolyses the β-1,4-glycosidic bonds of chitin, the structural polysaccharide of fungal cell walls (and of arthropod exoskeletons). In molecular biology, chitinase is the enzyme of choice for digesting fungal cell walls during DNA isolation: fungal cells, unlike bacterial or plant cells, resist lysozyme and cellulase because their walls are chitin-based. Treating the fungal pellet with chitinase releases the protoplast, after which standard SDS-based lysis liberates the DNA. The same role for cell-wall digestion is played by lysozyme (for bacteria) and cellulase (for plants).

(a) PCR amplifies a defined DNA region in vitro through denaturation–annealing–extension cycles using two primers and Taq polymerase; 30 cycles → ∼ 10⁹ copies. (b) Restriction enzymes are bacterial endonucleases that cut DNA at specific palindromic sites and are the cutting tools of gene cloning. (c) Chitinase digests the chitin in fungal cell walls during DNA isolation from fungi.

Concept used. Each pair contrasts two molecules / enzymes that are biochemically similar but functionally and structurally distinct. A well-formed answer is a side-by-side table of property : entity-1 : entity-2.

(a) Plasmid DNA vs Chromosomal DNA.

(b) RNA vs DNA.

(c) Exonuclease vs Endonuclease.

(a) Plasmid DNA = small, circular, extrachromosomal, self-replicating, dispensable; chromosomal DNA = large, essential, houses all life-support genes. (b) DNA = deoxyribose + A,T,G,C + ds-helix + stable + stores info; RNA = ribose + A,U,G,C + mostly ss + alkali-labile + executes info. (c) Exonucleases cut from ends, one nucleotide at a time; endonucleases cut internally at specific sites and generate two new ends.